On Tue, Jan 28, 2020 at 01:56:00PM -0700, Jens Axboe wrote: > On 1/28/20 1:50 PM, Pavel Begunkov wrote: > > On 28/01/2020 23:19, Jens Axboe wrote: > >> On 1/28/20 1:16 PM, Pavel Begunkov wrote: > >>> On 28/01/2020 22:42, Jens Axboe wrote: > >>>> On 1/28/20 11:04 AM, Jens Axboe wrote: > >>>>> On 1/28/20 10:19 AM, Jens Axboe wrote: > >>>>>> On 1/28/20 9:19 AM, Jens Axboe wrote: > >>>>>>> On 1/28/20 9:17 AM, Stefan Metzmacher wrote: > >>>>>> OK, so here are two patches for testing: > >>>>>> > >>>>>> https://git.kernel.dk/cgit/linux-block/log/?h=for-5.6/io_uring-vfs-creds > >>>>>> > >>>>>> #1 adds support for registering the personality of the invoking task, > >>>>>> and #2 adds support for IORING_OP_USE_CREDS. Right now it's limited to > >>>>>> just having one link, it doesn't support a chain of them. > >>>>>> > >>>>>> I'll try and write a test case for this just to see if it actually works, > >>>>>> so far it's totally untested. > >>>>>> > >>>>>> Adding Pavel to the CC. > >>>>> > >>>>> Minor tweak to ensuring we do the right thing for async offload as well, > >>>>> and it tests fine for me. Test case is: > >>>>> > >>>>> - Run as root > >>>>> - Register personality for root > >>>>> - create root only file > >>>>> - check we can IORING_OP_OPENAT the file > >>>>> - switch to user id test > >>>>> - check we cannot IORING_OP_OPENAT the file > >>>>> - check that we can open the file with IORING_OP_USE_CREDS linked > >>>> > >>>> I didn't like it becoming a bit too complicated, both in terms of > >>>> implementation and use. And the fact that we'd have to jump through > >>>> hoops to make this work for a full chain. > >>>> > >>>> So I punted and just added sqe->personality and IOSQE_PERSONALITY. > >>>> This makes it way easier to use. Same branch: > >>>> > >>>> https://git.kernel.dk/cgit/linux-block/log/?h=for-5.6/io_uring-vfs-creds > >>>> > >>>> I'd feel much better with this variant for 5.6. > >>>> > >>> > >>> To be honest, sounds pretty dangerous. Especially since somebody started talking > >>> about stealing fds from a process, it could lead to a nasty loophole somehow. > >>> E.g. root registers its credentials, passes io_uring it to non-privileged > >>> children, and then some process steals the uring fd (though, it would need > >>> priviledged mode for code-injection or else). Could we Cc here someone really > >>> keen on security? > >> > >> Link? If you can steal fds, then surely you've already lost any sense of > > > > https://lwn.net/Articles/808997/ > > But I didn't looked up it yet. > > This isn't new by any stretch, it's always been possible to pass file > descriptors through SCM_RIGHTS. This just gives you a new way to do it. > That's not stealing or leaking, it's deliberately passing it to someone > else. I've been reading along quietly. In addition to what Jens said, to ease everyone's mind: pidfd_getfd() doesn't allow to unconditionally grab file descriptors for any task. That would be crazy. The calling task needs ptrace_may_access() permissions on the target task, i.e. the task from which you want to grab the io_uring file descriptor. And any calling task that has ptrace_may_access() permissions on the target can do much worse than just grabbing an fd. Christian
